1. Field of the Invention
The present invention relates to enameled insulated wires formed by applying an insulation varnish around a metal conductor followed by baking, and particularly to enameled insulated wires and, in which adhesion between the metal conductor and the insulation coating does not significantly degrade even at relatively high temperatures. Furthermore, the present invention relates to manufacturing methods of the enameled insulated wire.
2. Description of Related Art
Enameled insulated wires are widely used for coil wires in electrical equipment such as rotary electric machines and transformers. Such enameled insulated wires include: a metal conductor, which is formed so as to have a desired cross section (such as circular and rectangular) depending on a shape and application of the coil; and a single layer or multilayer insulation coating around the metal conductor. Typically, enameled insulated wires (sometimes simply referred to as “insulated wires” and “enameled wires”) are formed by applying, around a metal conductor, an insulation coating varnish (also referred to as “insulation varnish”) followed by baking. Such insulation varnishes are typically prepared by dissolving a resin (such as polyimide, polyamideimide, and polyesterimide) in an organic solvent.
In recent years, in order to reduce the manufacturing cost of such electrical equipment as described above, there has been a trend toward automated, high throughput manufacture. In keeping with this trend, progress is also being made toward the automation of coil winding processes. And, because of the downsizing of such electrical equipment, enameled insulated wires are wound around a smaller diameter core of the coil with a finer pitch under higher tension. During such processes, enameled insulated coil wires are subjected to stronger bending force or higher friction, and therefore have a higher possibility of being damaged. Such damage to the enameled insulated coil wire may cause failures such as inter-layer short circuits and ground faults, thus degrading the yield of the coil. Hence, there is a strong demand for enameled insulated wires with better processability.
To improve the processability of enameled insulated wires, various efforts are being made, such as increasing their resistance to mechanical stresses and providing their surface with lubricity. However, such efforts have yet to sufficiently meet recent harsh demands of coil winding processes. So, in another effort, attempts are being made to increase adhesion between metal conductors and insulation coatings. Such an increase in adhesion can prevent the insulation coating of an enameled insulated wire from peeling from the metal conductor even when the wire receives larger external stresses. Thus, better processability is obtained.
A typical method for achieving stronger adhesion between a metal conductor and an insulation coating is to improve the insulation varnish used to form the insulation coating. For example, JP-A Hei 10 (1998)-334735 discloses an insulated wire formed by applying a polyimide-based insulation varnish around a copper conductor followed by baking, in which the polyimide-based insulation varnish used is prepared by adding 0.1 to 20 parts by weight of melamine to 100 parts by weight of a polyimide-based resin. According to this JP-A Hei 10 (1998)-334735, the resulting polyimide-based insulation coating has excellent properties such as high mechanical strength, high thermal resistance and high chemical resistance and also achieves very strong adhesion to the copper conductor. Such a good result is obtained probably because incorporation of terminal groups with high polarity such as a hydroxy group and an amino group in the insulation coating enhances interaction between the copper conductor and the insulation coating.
Another method for achieving stronger adhesion between a metal conductor and an insulating coating is to surface-treat the metal conductor to form an intermediate layer and to improve the adhesion by the aid of this intermediate layer. For example, JP-A 2001-93340 discloses an insulated wire formed by precoating a core wire with an alkoxysilane compound and then by coating a thermoplastic polyester-based resin or a resin composition containing it around thus precoated core wire. This method is effective probably because mercapto groups in a mercapto-alkoxysilane or amino groups in an amino-alkoxysilane form a strong chemical bond with a copper conductor, and also condensation of silanol groups in these compounds causes the precoating (intermediate) layer to strong adhere to both the core wire and the resin coating.
On the other hand, recently, there has been a growing demand for high output power and/or low energy consumption as well as small-size in the electric apparatus field. To meet this demand, a rapid trend exists toward use of inverters to control rotary electric machines. Also, in such applications, higher voltage and higher current (i.e., higher electric power) inverters are increasingly used. As a result, coils are increasingly used at higher operating temperatures.
Conventional methods for achieving high quality resins have a problem in that even if a particular property of interest of a resin is improved, other important properties thereof may be sacrificed. In addition, even if all important properties of a resin are improved, a cost problem arises because such a novel resin may require non-conventional special manufacturing processes which conventional mass production lines do not include. Such above-mentioned intermediate layers can also be formed by using silane coupling agent treatment. However, the intermediate layers formed by silane coupling agent treatment are prone to suffer from thermal degradation at high temperatures (e.g., above 150° C.) particularly when kept for a long time (e.g., one hour), thus potentially degrading adhesion between the metal conductor and the insulation coating. That is, intermediate layers formed by silane coupling agent treatment have a problem of poor thermal reliability (poor long term thermal resistance).